1. Field of the Invention
This invention relates to pressure sensitive recording sheets.
2. Description of the Prior Art
Pressure sensitive record materials are known. For example, a first sheet, generally paper, is treated on one side with a coating of microcapsules containing a solution of color former. A second sheet is treated on one side with a developer (coreactant material) such as acidic clay or polymeric material. When the coated side of both sheets are brought into contact with each other and pressure applied to the first sheet, e.g., by pencil, pen or typewriter, the microcapsules are ruptured and the color former is released and comes into contact with the second sheet containing the developer. A color is produced by reaction between the color former and the developer.
Several disclosures relating to pressure sensitive recording paper which are particularly directed to the selection of the solvent for the color former are set forth below.
U.S. Pat. No. 3,836,383--Kiritani et al--Sept. 17, 1974 discloses as the solvent two alkyl substituted benzene nuclei connected by --C.sub.n H.sub.2n -- and --CH(CH.sub.3)--.
U.S. Pat. No. 3,936,566--Sato et al--Feb. 3, 1976 discloses as the solvent the reaction product of styrene and alkyl substituted aromatic hydrocarbons.
U.S. Pat. No. 4,130,299--Wygant--Dec. 19, 1978 discloses benzylated xylenes as the solvent.
One of the problems confronting the art is the selection of the solvent for the color former. A number of criteria relating to the selection of the solvent in manufacture of pressure sensitive paper are:
The solvent must dissolve the color former.
The solvent must have reduced viscosity.
The solvent must be non-toxic.
The solvent must have minimal odor.
The solvent must not inhibit color development.
As the art has demonstrated, there is no one universal or completely acceptable solvent. All, to some degree have undesirable properties such as toxicity, odor, inhibition of color development, etc. Further, as pointed out in U.S. Pat. No. 4,130,299 there still remains a lack of understanding of routes to odor improvement. This is borne out by the fact that the prior art has approached the problem by depending upon modifications of the solvent which are position isomers or which introduce additional alkyl groups on known hydrocarbon moieties.